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Structural design for fire safetyWhere have we come from? & where are we going?
Dr. Danny Hopkin – Associate Director
London Fire Brigade – July 2016
Scope
• Fire resistance – why?• Future trends – where next?• Design at the interface – an
alternative• Summary• Questions
Fire resistance - HistoryModern application, dated origins
Why do we have FR?
• Great conflagrations in the late 1800s and early 1900s;
• Fire resilient buildings became a social expectation;
• ‘Fire proof’ materials emerged;• A lack of trust in private testing;• A need to independently verify;• Emergence of federal &
municipal fire test laboratories
The start of ‘standard’
• Earliest references of a ‘standard fire test’ – New York – late 1800s;
• Five hours at 2000°F;• Post Baltimore - “no ordinary room
would have enough inflammable material in it to maintain a 1700°F fire for more than 30 minutes”;
• Anecdotal FF experience;• Still ‘non-standard’.
A benchmark test• Ira Woolson - NFPA -
“unify all fire tests under one single standard and remove an immense amount of confusion within the fire testing community” (1917);
• By the 1920s the time-temperature curve is standardised and fire resistance is born;
A concept discredited
Ingberg 1928
Relates real fire severity to equivalent ‘standard’ durations
Woolson 1917 NFPA meeting –
“We want to get it as nearly right as possible before it is finally adopted, because, after it is adopted by these various associations, it will be pretty hard to change it”
Woolson’s premonition
Fast forward a century
Structural FR - Defined
• Tests whether an isolated structural element does not violate particular performance criteria after a set period of time in a furnace, when subject to the standard time-temp curve;
• Deflection limit span/20;• It cannot ever be a measure of
survivability in a real fire; • The standard fire is not a
standard fire, it’s not even a fire!
• Energy flow in is balanced against the losses to achieve ‘the standard fire curve’
Is it a ‘fair’ test?
Pump some energy in
Lose some energy to the furnace walls
Energy absorbed into the specimen
A concrete slab
A CLT slab
• Less energy is required to balance the losses because the specimen is contributing
Is it a ‘fair’ test?
Pump some energy in
Lose some energy to the furnace walls
Energy absorbed into the specimen
Specimen produces energy as it burns
Future trends, divergenceand the renaissance of a familiar foe
Going up & urban
Timber renaissance
Sustainability
• 400+ towers (>20 storeys) proposed in London…
• There will be features that are ‘unusual’ or sensitive to fire…
• How will we approach their design?
• Wind – performance based assessment
• Seismic – performance based assessment
• Fire?............................
Lame substitutions*
Fire safety engineering
Structural engineering
Structural design for fire safety
*Credit – Guillermo Rein
The 1st kind
Struct. engineer is replaced by pseudo-science
Fire safety engineering
Failure at x°C
Fire eng. replaced by pseudo-science
Structural engineeringTime
Tem
pera
ture
Failure at x mins
The 2nd kind
Both eng. replaced by pseudo-science
Time
Tem
pera
ture
Failure at x°C
Sound familiar?
The 3rd kind
Solution – protect all steel members to a 120 minute standard for a limiting temperature of X°C
Engineering…..done
• “intended to provide guidance for the more common building situations…”
• “need to take into account the particular circumstances of the individual building…”
A health warning
Apathy?
1940s
1990s
2020s
Familiar magic numbers
Design at the interfaceA structural fire engineering strategy for an expressed Cor-Ten frame
Fire safety engineering
Structural engineering
Structural fire engineering
The interfaces
What?
Who? How?
An interface between disciplines
The interfaces between facets of a successful
delivery
Regulations
Responsibility Skill & Care
Structural engineers understood they were responsible for ensuring “stability for a reasonable period” in fire
Those responsible for construction were engaged at an early stage and became familiar with the requirements
Design team understood that the fire performance demands were beyond their competency & delegated
Competence – A prerequisite for success
4 Pancras SquareFor an industrial building
An industrial site
The building• A 10 storey office – 46 m in height;• Predominantly a concrete frame – cast
insitu & PT;• Architectural feature – external Cor-Ten
frame;• A huge Cor-Ten transfer structure;• Tricky interfaces.
A successful solutionA melange of competing goals, obligations & constraints, of varying intelligibility
The life safety goal
• "Stability for a reasonable period";• Consistency of risk – Kirby, et. al;• Overall reliability requirement of 97%;• Active reliability contribution of 93%;• Passive reliability requirement of 49%;• All 50% have the potential to fully
develop.
0 20 40 60 80 100 120 140 160 180 2000
20
40
60
80
100
Height (m)
Frac
tile
(%)
Fire manifestation
0%
20%
40%
60%
80%
100%
Peak steel temperature (°C)
Perc
entil
e (-)
Thermal conditions
• A lack of guidance – Law & O’Brien – SOA;
• Steady state analysis – overly conservative;
• A need to quantify transient behaviour;
• Consider the impact of wind;• Quantifying thermal gradients,
etc., key.
Thermal conditions
Side 1 Side 2 Rear Front0.00
0.20
0.40
0.60
0.80
1.00
Elevation of elementRela
tive
prop
ortio
n of
com
part
men
t te
mpe
ratu
re (-
)
0 30 60 90 120 150 180 2100
200
400
600
800
1000
1200Fire CompartmentSidesRearFront
Time (min)AS
T (°
C)
- BS EN 1991-1-2 Annex B as a ‘scalar’- Benchmarked against CFD models- Adequately conservative.
Element orientation influences exposure:
• Location ‘manages’ exposure;
• Sections still very hot;• Concrete filling, where
practicable;• Shielding, where
permissible; &• Otherwise, plate sizing.
Fire Floor
Floor Above
Managing temperature
Materials – Cor-Ten
• Cor-Ten is not a typical material;
• The scale of the section is not typical;
Structural response
• Two key areas:• Vierendeel transfer; &• Columns
• Other complications:• Connections;• PT;• Bi-metallic corrosion & PFP.
Vierendeel behaviour
0 5000 10000 15000 20000
-4000000
-3000000
-2000000
-1000000
0
1000000
2000000
3000000
Time (s)
Axia
l for
ce (k
N)
• Expansion governed;• Very sensitive to TFs;• Doesn’t deflect excessively;• Plastic strain tension;• A building that needs to ‘breathe’;• Matching ‘actual’ vs. ‘idealised’.
Column behaviour
• Concrete filling;• Explored rebar vs. T;• T more ‘buildable’;• UC 254x127x84 (S355);• Actions influenced by
curvature & slab ‘push-out’;• Sensitivity to vertical fire
spread explored;
0 30 60 90 120 150 180
-150%-100%
-50%0%
50%100%150%
BF WEB_CTF MAXMin
Time (min)Inne
r Tee
Util
isati
on (%
)
Lessons & key points
• Struct. Eng. understood their responsibility & limits;• “Stability for a reasonable period” not FR120 + sprinklers;• They understood the expertise req’d & delegated;• Those responsible for delivery were involved in design.
• Quantification of the goal -> rational basis -> rational process; • Thermal tools are inadequate for external exposure;• Cor-Ten does not behave like regular carbon steel;• Bigger is not always better.
Design by convention
Skill & care
• Successful fire engineering doesn’t end when a report is issued….
SummaryConsidered structural design for fire safety
Final remarks• Fire resistance is ‘old hat’;• Prescriptive guidance caters for
the simple;• The legal requirement is stability
for a reasonable period not a predefined level of FR;
• An approach commensurate with complexity;
• Competence is a prerequisite for successful design;
• A great report ≠ a great solution.
"If you always do what you've always done, you'll always get what you've always got.“
H. Ford
Thanks for your time• [email protected]
• http://uk.linkedin.com/in/dannyjhopkin
• https://twitter.com/DannyHopkin
• http://www.slideshare.net/DannyHopkin